1. Field of the Invention
The invention relates to a Fabry-Perot device, and more particularly, to a Fabry-Perot device capable of compensating for an error of full width at half maximum (FWHM) in fiber optical communication applications and the manufacturing method thereof.
2. Description of the Related Art
During the recent years, the characteristics namely reflection, refraction, interference, and fast transmission rate of light waves, have flourished various optical application techniques. And among them all, the development of optical communication surpasses the rest. Because the optical communication uses the traveling of light waves to transmit data, it is without doubt that the transmission and reception efficiency depends largely on the characteristics of light waves during the data transmission. In other words, to achieve the expected transmission and reception efficiency, these various active or passive optical devices being used in the current optical communication network must overcome restrictions set by the characteristics of light waves.
To satisfy such requirement, the manufacturing precision of existing optical devices without exception are manufactured by sub-micron, or even nanometer manufacturing techniques like semiconductor and micro-electro-mechanical systems (MEMS) manufacturing techniques. Take Fabry-Perot resonant cavities (or interferometers) for instance, they are developed by semiconductor techniques and the surface micro-machining techniques, and are also being extensively applied in the fields of optoelectronics, mechanics, biomedical as well as environmental detections.
FIG. 1 is a schematic diagram showing a prior Fabry-Perot Etalon. Referring to FIG. 1, a Fabry-Perot Etalon 10 includes two parallel planar mirrors 11 and 12 having reflectance R. Both the planar mirrors 11 and 12 are able to partly reflect an incident light 13, and a distance Dop between them is defined as the optical thickness. When an incident light with wavelengths λ1˜λn enters the Fabry-Perot Etalon 10, the incident light goes back and forth between the planar mirrors 11 and 12 due to the reflection effect of the incident light 13 on the planar mirrors 11 and 12; and only the outgoing light 14 with wavelength λi may pass through under the adjustment of the distance Dop between the planar mirrors 11 and 12, thereby achieving a filtering effect. The optical properties of a Fabry-Perot device are defined by the following equations:Free spectrum ratio, FSR=(λ2)/2n Dop;where λ is the center wavelength, n is the optical index, and Dop is the distance between the two planar mirrors;Finesse, F=π√{square root over ( )}R/1−R; where R is the reflectance of the two planar mirrors; andFWHM=FSR/F. 
For the reason that the wavelength distribution of the outgoing light 14 passed through the Fabry-Perot Etalon 10 is almost a Gaussian distribution, a designer consequently takes the FWHM value of a filtered light wave as the prime design parameter in the application of common optical communication systems. With respect to the Fabry-Perot Etalon 10 mentioned above, the reflectance of the two planar mirrors 11 and 12 along with the optical thickness Dop dominate the FWHM value of the wavelength distribution of the outgoing light 14. Therefore, it is the designer's primary task to control the optical thickness Dop between the two planar mirrors and the reflectance R in manufacturing and configuring the two planar mirrors 11 and 12.
For example, the spectrum characteristics of the outgoing light 14 need to be satisfied with the condition of a FWHM being 0.37 nm and a free spectrum ratio (FSR) being at least 40 nm in order that the particular wavelength λi of the outgoing light 14 passed through the aforesaid Fabry-Perot 10 equals to a center wavelength λ, that is, 1550 nm, of the C band within the wavelength range 1530 nm ˜1565 nm, according to the ITU GRID 100 GHZ specifications in fiber optical communication. In this case, the finesse F has to be 108. FIG. 2 shows the definitions for the FWHM, the FSR and the finesse in this case. Furthermore, it is calculated that the optical thickness Dop between the planar mirrors 11 and 12 turns out to be 30 μm at most according to the relationship between the FSR, the center wavelength λ, the optical thickness Dop and the medium reflectance n (FSR=λ2/2nDop). It is also calculated that the reflectance R of the two planar reflecting mirrors 11 and 12 turns out to be 0.97 at least according to the relationship between the finesse F and the reflectance R of the two planar mirrors, that is, F=π√{square root over ( )}R/1−R.
Nevertheless, referring to FIG. 3, considering a prior Fabry-Perot 20 resonant cavity manufactured by current MEMS and semiconductor manufacturing techniques, it is common to etch a flute of a particular depth on a glass substrate 21, on which a fixed reflecting surface 23 is formed by coating a layer of optical thin film, and a mobile reflecting surface 24 coated with an optical thin film is formed on a silicon substrate 22 by using MEMS manufacturing techniques. In addition, the distance d in which the mobile reflecting surface is capable of moving is comparatively small than the distance D between the two reflecting surfaces, that is d<<D. With respect to the above, the reflectance of the two reflecting mirrors 23 and 24 is actually decided by the quality of the optical coatings, and the mature optical coating techniques in current use are merely capable of controlling the reflectance tolerance of the two reflecting surfaces within ±1%. Hence, in the above example, the reflectance of the optical coatings reaches practically 0.97±0.01, that is, 0.96˜0.98. The finesse F, 77˜155, and the FWHM value, 0552˜0.258 nm are obtained by substituting the reflectance R, 0.96˜0.98, into the finesse equation (F=π√{square root over ( )}R/1−R) and the FWHM equation (FWHM=FSR/F). However, such FWHM tolerance is too large and almost inapplicable. Thus, the actual FWHM value largely disagrees with the expected value due to the practical tolerance of the optical coatings for the prior Fabry-Perot resonant cavity 20. Therefore, the distance D between the two reflecting surfaces shall be adjusted for compensation so that the FWHM matches with the designer's expected value. Take FWHM being 0.37 nm for example, we have the result that F is 108, FSR is 40 nm, and D is 30 μm from the equation (FSR=FWHMXF) assuming R is 0.97. In addition, we have the result that F is 155, FSR is 0.37 nm, and D is 20.8 μm from the equation (FSR=FWHM×F) assuming R is 0.98. It is observed that the reflectance tolerance can be compensated by adjusting the measurement of D so that the FWHM value remains constant. However, during the manufacturing process, the glass substrate 21 and the silicon substrate 22 are fixed together, meaning that the distance D between the two reflecting surfaces in the Fabry-Perot resonant cavity 20 stays fixed. In this case, the prior Fabry-Perot resonant cavity is incapable of compensating the FWHM error caused by the tolerance of the optical coatings, consequently, the prior Fabry-Perot resonant cavity fails to meet the designer's requirements.
To solve the above issue, the designer of the present invention proposes a Fabry-Perot device that fulfills the expected FWHM value so that the Fabry-Perot device may be applied in a effective manner in the fiber optical communication.